Deck Footing & Post Calculator — Spacing, Size & Depth

Deck footing spacing equals the deck beam span — the beam between two posts is the span, so the posts (and their footings) sit at whatever distance the beam can carry. For built-up treated beams under a typical 40/10 psf deck that lands at 5–8 ft depending on beam size; a (2) 2×8 reaches about 6 ft, deeper or triple builds reach further. The calculator above shows the post count your spacing produces — tighten the slider and watch the footing count (and the beam's margin) grow together. Size the beam itself with the wood beam span calculator, then let its answer set this page's spacing.

Below the frost line, full stop — a footing that heaves once lifts the post, and the deck never sits flat again. Your building department publishes the local frost depth (0–60 in. across the US; the calculator's slider covers it), and IRC R403.1.4 requires bearing below it. Two additions matter: the footing must also bear on undisturbed or compacted soil (12 in. minimum embedment even in frost-free regions), and the hole should flare or bell at the bottom so the base area matches the diameter the calculator computed — a straight-sided hole in loose soil under-delivers the bearing area.

Deck blocks — the precast pyramid footings, including TuffBlock-type surface blocks — are legitimate for one specific case: low platforms. The working limits are a frame under 30 in. off grade, short post or block-to-beam bearing, per-post loads under about 1,500 lb, and soil that drains. Outside that envelope, blocks fail in two ways: they ride on the surface, so frost lifts them, and their small bearing pad concentrates load on soil the footing math assumed was generous. The calculator's block verdict applies exactly these limits — when it flips to concrete, it is the frost depth or the post load doing the flipping, not the brand of block.

Height decides before load does. A 4×4 notched to carry a beam fails at roughly 2,000 lb, and notching is how most 4×4 posts get used — a 6×6 with a rated post cap carries several times that without any notch. The modern deck standard is therefore simple: 6×6 posts for anything that stands more than about 2 ft above grade or carries more than a light platform's load, 4×4 reserved for grade-hugging platforms where the beam sits nearly on the block. Posts over 4 ft tall add diagonal bracing on freestanding decks — the calculator's recommendation line applies both rules to your numbers.

Most residential deck footing design uses 1,500 psf — the IRC's default assumption when no soil test exists — and the conservative answer is to keep using it unless you know better. Firm sand and gravel soils typically rate 2,000–3,000 psf and shrink the footing diameter an inch or two; soft clay can rate lower and grows the footing. The local building department knows your area's default value, and a garage slab's performance is a decent field test: if the neighborhood settles, assume 1,500 and round the footing up an inch. When in doubt, the cost difference between a 14″ and a 16″ hole is a few shovels of concrete.

The ledger deck's beam end still needs footings — every post that carries the beam gets one, frost depth and all, whether the post is 20 ft from the house or 20 in. The house side of an attached deck does not get footings because the ledger hands its share of the load to the foundation, which is one reason a ledger can never fasten to brick veneer, stucco, or a cantilever: those have no footing-grade load path behind them. Freestanding decks double the footing count because both beams stand on their own posts — the calculator's beam-layout switch shows the count flip.

Nothing except spelling and region — 'deck footer calculator' and 'deck footing calculator' describe the same buried concrete pad that spreads a post's load into the soil. 'Footer' is the older trades term and dominates casual search; 'footing' is what the code book and this page use. The post is the column above it, the pier (if used) is the concrete or block column between footing and post, and the hardware — post base, cap, anchor — connects the stack. All four layers belong in the schedule the calculator copies for you.

No — the tub's weight enters the deck through the joists either way, and the joists deliver it to beams, then posts, then footings. A filled 4-person spa adds 3,500–4,500 lb over its footprint; the posts under the tub zone see loads no standard footing schedule predicted. Either run this calculator with the 60 psf hot-tub preset and accept the footings it calls for, or drop a dedicated beam under the tub with its own posts and footings sized exactly for the tub zone. What never works is hoping the tub's plastic feet 'spread it out' — they spread it into the decking, which forwards every pound to the same footings.